More and more vehicle structural members that used to be fabricated from separate welded parts, such as engine cradles, are now being designed as hydroformed tubular members in which all, or most, of the parts are integrated. In a typical hydroforming operation, a cylindrical metal tube of constant wall thickness is bent into the overall shape desired for the final part, then placed between a pair of dies. The dies provide a cavity around the tube which has an interior shape matching the exterior shape desired for the part. Then, the ends of the tube are sealed and it is internally pressurized to expand it into the shape of the dies' cavity. Despite some small percentage of stretch that may occur as the tube expands, the wall thickness in the final part still basically matches that of the original tube or blank, and is therefore substantially constant at all points.
In some circumstances, a part that is locally thickened or reinforced for extra strength is desired. U.S. Pat. No. 4,759,111 discloses a method for making such a part in which a short sleeve is pushed concentrically over the main tube before it is expanded. The main tube is forced out and into the sleeve, which is left firmly fixed in place as a reinforcement. Another method is disclosed in co-assigned U.S. Pat. No. 5,333,775, in which several tubes of differing thickness are butt welded together, and then hydroformed. In both designs, extra welding or attachment steps are needed, beyond the basic process of simply placing a one piece tube blank into the dies. Also, in both cases, at any given cross section, the wall thickness as one moves circumferentially around the cross section is basically constant. That is, if a particular cross section of the final part is basically rectangular with four walls, each of the four walls has the same thickness, even though the wall thickness of cross sections at other points along the length of the part may be different.